1. Field of the Invention
The present invention relates to an ultrasound diagnosis apparatus which acquires Doppler-mode image frames in a Doppler-mode. The present invention also relates to a method of ultrasound scanning in a Doppler-mode.
2. Discussion of the Background
In a typical ultrasound diagnosis apparatus, transducers built in an ultrasound probe generate ultrasound pulses towards a patient body. The transducers also receive echo signals returning from the patient body as a result of the ultrasound pulse generation. The echo signals occur due to a difference of acoustic impedances among tissues of the patient body. The received echo signals are displayed as ultrasound images on a display monitor. Since the ultrasound diagnosis apparatus requires only simple and easy operations such as contacting the ultrasound probe with a surface of the patient body for acquiring the ultrasound images (e.g., a real-time two-dimensional ultrasound images), the ultrasound diagnosis apparatus is widely used for functional and/or morphological diagnoses of various organs of the patient.
An ultrasound pulse echo technique and an ultrasound Doppler technique have been developed as major techniques in the field of ultrasound diagnoses. These two techniques contribute significantly to a progress in obtaining patient body information based on echo signals returned from various organs or blood cells of the patient. Recently, B-mode images acquired by the ultrasound pulse echo technique and color Doppler images (or Doppler-mode images) acquired by the ultrasound Doppler technique are usually used in ultrasound image diagnoses.
In a color Doppler technique as an example of the ultrasound Doppler technique, a predetermined cross section inside the patient body is scanned by ultrasound pulses. When the ultrasound pulses are insonified to moving reflectors such as, for example, blood (blood cells), Doppler-mode images are acquired in accordance with Doppler frequency shifts caused in correspondence with speeds of the reflectors (e.g., blood flow velocities). In the past, the color Doppler technique was used to image blood conditions in a heart chamber where blood flows fast. Recently, however, it is also possible to apply the color Doppler technique to image a very slow blood flow such as, for example, a tissue blood flow in an abdominal organ.
When a speed of the moving reflector is measured based on the Doppler frequency shifts of the moving reflector, an ultrasound transmission and reception is repeated n (n>1) times on the moving reflector at a rate interval Tr. The speed of the moving reflector is measured based on a series of n echo signals resulting from the repetition of the ultrasound transmission and reception. In this measurement, a measurable minimum velocity Vmin of the low-speed moving reflector depends on a frequency resolution Δfd in a frequency analysis conducted on the series of n echo signals. The frequency resolution Δfd may be defined by the following formula (1) when a repetition frequency of the ultrasound transmission and reception (hereinafter referred to as a rate frequency) is expressed by fr (fr=1/Tr).Δfd=fr/n  (1)
As may be understood by the formula (1), to improve the measurable minimum velocity Vmin, it is necessary to lower the rate frequency fr. Alternatively, it may be necessary to increase the repletion number of times n of the ultrasound transmission and reception in a predetermined direction. In addition, a real time performance required for ultrasound images may be determined by the number of image frames to be displayed in a unit time (hereinafter referred to as a frame frequency) Fn. The frame frequency Fn is expressed by the following formula (2).Fn=fr/n/m=Δfd/m  (2)
The m is the total number of scan lines which are necessary to construct one image frame. Since the frame frequency Fn and the measurable minimum velocity Vmin contradict each other, it was difficult to keep both of them in predetermined conditions at the same time. Some improvement of scan techniques, however, is provided to reduce the problem mentioned above. Such improved scan technique is disclosed, for example, in Japanese Patent Application Publication No. PS64-43237 and hereinafter referred to as an interleaving scan technique.
FIG. 1A is an illustration showing one of the interleaving scan techniques. FIG. 1B is an illustration showing another one of the interleaving scan techniques. In both FIGS. 1A and 1B, a plurality of transmission and reception directions corresponding to the scan lines (or ultrasound beam directions) (hereinafter referred to as raster directions) R1 to Rm in a sector scan are shown in upper stands. One ultrasound beam may be generated in one raster direction. The order of the ultrasound transmission and reception with respect to raster directions is shown in lower stands.
In the technique shown in FIG. 1A, the ultrasound transmission and reception is conducted in a raster direction R1 at time t1. The ultrasound transmission and reception is then conducted in a raster direction R2 at time t2. Also, the ultrasound transmission and reception is conducted in a raster direction R3 at time t3. Similarly, a set of the ultrasound transmission and reception in the raster directions R1 to R3 is conducted at times t4 to t6 and times t7 to t9. Therefore, as described above, the ultrasound transmission and reception is repeated n times (here, e.g., n=3) in each raster direction. That is, for example, the ultrasound transmission and reception is conducted in the raster direction R1 at times t1, t4, and t7. The ultrasound transmission and reception are repeated n times in every group of a predetermined number Q (here, e g., Q=3) of raster directions. The ultrasound transmission and reception is conducted at an interval Tr.
In the technique shown in FIG. 1B, the ultrasound transmission and reception is conducted in a raster direction R1 at time t1. The ultrasound transmission and reception is, however, not conducted in any raster direction at times t2 and t3. The ultrasound transmission and reception restart in the raster direction R1 at time t4. At time t5, the ultrasound transmission and reception is conducted in a raster direction R2. At time t6, the ultrasound transmission and reception is not conducted. The ultrasound transmission and reception are then conducted in raster directions R1 to R3 at times t7 to t9, respectively. Further, the ultrasound transmission and reception are conducted in raster directions R1 to R4 at times t10 to t13, respectively. Next, the ultrasound transmission and reception are conducted in raster directions R2 to R5 at times t14 to t17, respectively. After this, the ultrasound transmission and reception are repeated in a manner similar to the above description until a raster direction Rm. That is, the ultrasound transmission and reception are conducted n (here, e.g., n=4) times at an interval Ts (here, e.g., Ts=3Tr) in one raster direction. The raster direction is shifted one by one.
According to the techniques shown in FIGS. 1A and 1B, the frequency resolution Δfd which determines the measurable minimum velocity Vmin is expressed by the following formula (3).Δfd=fs/n=fr/Qn  (3)
The fs (fs=1/Ts) is a repetition frequency of the ultrasound transmission and reception in each raster direction. This repetition frequency fs is one third of the rate frequency fr. The frame frequency Fn is constant. Therefore, it is possible to triplicate the measurable minimum velocity Vmin without lowering the frame frequency Fn.
However, since one Doppler-mode image frame is prepared based on the ultrasound transmission and reception in the raster directions R1 to Rm according to the technique shown in FIG. 1A, the prepared Doppler-mode image frame includes discontinuous boundaries between groups of the Q raster directions (e.g., a group of the raster directions R1 to R3 and a group of the raster directions R4 to R6) (i.e., a boundary between the raster directions R3 and R4). This is because of a time phase difference between, for example, image data prepared based on the ultrasound transmission and reception in the raster directions R1 to R3 and image data prepared based on the ultrasound transmission and reception in the raster directions R4 to R6. Therefore, the more the number of n and/or Q increases, the more noticeable the boundary discontinuity becomes in the prepared Doppler-mode image frame. Also, as the number of the groups of the raster directions increases, the boundary discontinuity appears more frequently in the prepared Doppler-mode image frame. The boundary discontinuity may particularly be likely to occur and become noticeable, for example, when a pulsant blood flow is imaged, when tissues around blood vessels are subject to a respiratory movement and/or are affected by a heart beat, and when the ultrasound probe is moved by an operator. Such boundary discontinuity disturbs ultrasound image diagnoses.
According to the technique shown in FIG. 1B, scan controls become complicated. In addition, when a color Doppler image which has a shallow perspective depth is displayed together with a B-mode image which has a deep perspective depth, a display rate frequency is required to be unified to be a rate frequency for the deep perspective depth image. Therefore, this technique has a problem of a severely lowered frame frequency Fn.